1. Field of the Invention
The instant disclosure relates to a light sensor; in particular, to an ambient light sensor.
2. Description of Related Art
Ambient Light Sensor (ALS) provides outputs that approximate human eye response. It is very useful in display brightness management with the purpose of extending mobile battery life and providing optimum viewing experience in diverse indoor and outdoor lighting conditions.
In mobile phone application, it is very common to use tinted or black glass to improve phones outlook. However, black glass presents a particular challenge for ambient light sensors as it distorts the spectrum of ambient light that falls on the ambient light sensor. Black glasses typically have the transmission rate as shown in FIG. 1. It has much higher transmission rate for the IR light spectrum, to which is not detectable by human eye, than the visible light spectrum.
By hiding under the black glass, the sensor would need very wide dynamic range while maintaining reasonable resolution to detect both indoor lightings (primarily fluorescent or white LED that has very low IR content) and outdoor lightings (primarily sunlight that has very high IR content).
Existing implementation to extend sensor dynamic range is described as follows. Firstly, the sensor ADC output is linear to incident light in logarithmic domain. Secondly, the sensor with higher ADC resolution should be utilized. For example, instead of 16 bit ADC, an 20 bit ADC would increase the detection dynamic range by 16×. However, the earlier implementation has degraded the sensor resolution significantly while the later solution has increased the ADC conversion time proportionally that effectively reducing the sensor conversion speed.